1. Field of the Invention
This invention relates to reactors and to reactor systems in general, and more specifically, to a mixer/contactor for a slurry reactor system. The mixer/contactor is advantageous for slurry processes wherein contact time may be relatively long, and/or wherein minimum energy input for suspending and mixing the slurry is desired, for example, in the biological remediation of contaminated sludges or soils.
2. Related Art
Slurry reactors are commonly used for processing ores, soils, and wood chips. Also, they are commonly used to effect the biological, enzymatic or chemical conversion of soluble and insoluble reactants. A slurry is a mixture of a liquid and solid particles, wherein at least a portion of the solid particles are suspended in the liquid. Especially, there is a large need to remediate sludges or soils contaminated with chemicals. These chemicals may be organic or inorganic, and hazardous or toxic. Many millions of cubic feet of soils contaminated with these chemicals exist throughout the world.
Biodegradation of many of these contaminating chemicals has been conducted. "Biodegradation" means breaking down these chemicals to less hazardous or less toxic reaction products via biological pathways using microorganisms. The microorganisms may operate aerobically or anaerobically. Also, the microorganisms may operate via oxidative pathways or reductive pathways. Microorganisms include bacteria, protozoa, fungi and algae. Biodegradation of soils contaminated with chemicals is one way to remediate the soil.
Often, the remediating microorganisms operate on the contaminating chemicals in a slurry environment in a reactor vessel, wherein the soil is mixed with water to at least partially suspend the soil particles for intimate contact with the microorganisms. To further increase suspension, mixing and contacting, a gas, such as air in aerobic applications, for example, may be added to the reactor vessel. When the reactor vessel contains microorganisms and a slurry, it is referred to as a bioslurry reactor.
Presently, at least four bioslurry reactor systems are being commercialized for soil remediation. The first system, developed by MOTEC, Inc. of Mt. Juliet, Tenn., involves technology adapted for treatment of pesticides, PCB's, dioxin and halogenated and nonhalogenated organic compounds. While demonstrated to be effective for treating sludge, liquids and soils having high organic concentrations, the MOTEC process has been reported to be less suitable for use with inorganic-laden wastes.
The MOTEC technology, which is a sequential process, is also referred to as liquid solid contact digestion (LSCD). The system involves two to three tank digestors which are aerated using air spargers and are agitated using turbine mixers. Alternatively, this technology may be adapted, by use of high shear propeller mixers, to enhance aerobic biological degradation in lagoons.
The second technology, developed by Detox Industries, Inc. of Sugarland, Tex., is intended for use in treating chlordane, myrex, oil, phenolics, polycyclic aromatic hydrocarbons, creosote, pentachlorophenol (PCP) and polychlorinated biphenyls (PCB's). The Detox system includes an open-topped reaction tank or on-site created lagoon that utilizes a synthetic liner. The tank is adapted to retain a slurry and is fitted with air distributors.
Another bioslurry reactor, consisting of several agitated and aerated vessels, has been used in a pesticide spill application by EOOVA of Redmund, Wash.
The MOTEC, Detox, and ECOVA systems described above are operated in batch mode. After the placement of contaminated soil and water into the reactor vessel, the vessel is aerated until a desired residual contaminant level is reached, and then the supernatant water is usually recycled and the slurry is discharged. Due to the ongoing aeration in these systems, many volatile organic substances are not biodegraded but rather are air-stripped. Some systems treat these air-stripped volatiles in a carbon adsorption filter whereas other systems simply discharge them to the atmosphere.
A fourth system, known as the EIMCO Biolift.RTM. system, utilizes a bioreactor that is a tank having a bottom, upstanding walls fixedly mounted thereon and a sealed top or cover, and which is adapted to receive and contain a slurry. The tank is fitted with a mechanical mixing means that operates to effectuate agitation and suspension of the particulates within the slurry housed within the tank. An air supply operates to provide oxygen, which is a necessary component of the biooxidation reaction taking place within the bioreactor. The air supply also is configured to provide suspension of the particulates within the slurry liquid housed within the tank. In addition, an air lift is provided for recirculating particulates which may have settled out of the slurry. The Biolift.RTM. system may be operated in continuous mode by using a screening device and exit conduit located near the top inside the tank.
Considerable literature is available describing slurry reactors for municipal and farm sewage digestion, but the total solids for these applications are usually below 10 wt %. The density of sewage sludges is much closer to the density of water than is the density of soil, and therefore the mixing method and design of these sewage sludge stirred reactors can be significantly different than that of soil-slurry reactors. Many sewage digester designs are unstirred, and the predominant mixing mechanism is the CO.sub.2 and CH.sub.4 gas generated in the reactor. The mixing occurs as these gas bubbles rise through the slurry. Propeller type mixers are sometimes added for more thorough mixing and to try to maintain the solids in suspension. The current design of most soil-slurry reactors is to finely pulverize the material and try to keep it in suspension with significant power input to shaft stirrers, aerators, recirculation pumps or a combination of these methods. The alternative approach is to not mix at all or to mix only occasionally. With the extended residence time required for most biodegradation, there is probably no need for a high shear or complete suspension agitation, especially for an anaerobic design.
In aerobic soil-slurry reactors it is difficult to maintain high oxygen concentrations due to the tendency for gas bubbles to coalesce. Also, since the reactors are usually low in profile, there is a very short liquid-gas contact time and a small surface area to volume ratio of the bubbles. Mechanical agitation is usually required to disperse gas bubbles and give smaller gas bubbles, but as the solids concentration increases the agitation effect decreases.
Common to all hazardous waste treatment systems utilizing microorganism activity is the requirement of providing an adequate supply of nutrients to the microorganisms. This provision allows biomass growth and facilitates the occurrence of biochemical reactions. Various approaches have been used to optimize bioactivity level in reactor vessels. In those systems wherein a multiplicity of connected reactor vessels have been suggested, e.g. cascade systems, a common problem is the retention and maintenance of biomass in a given reactor as effluent from the reactor is directed to the next reactor.
The clean-up of hazardous waste sites requires innovative approaches that am cost effective. Biological systems can play an important role in soil bioremediation, as they have in the field of wastewater treatment. In order to be cost effective in contaminated soil treatment, however, bioreactor vessels and processes are needed that can handle high solids concentrations and large throughput volumes with a minimum of input and/or operating energy.